Recent development of thin downsized secondary battery requires strongly a low-profile thermal fuse. Because current portable devices such as cellphones, notebook computers or video cameras and the like mainly adopt high capacity lithium-ion or lithium-polymer secondary batteries used in conjunction with thermal fuses.
A typical thermal fuse is composed of fusible alloys having low melting temperature.
A known conventional thermal fuse is disclosed in the Japanese Patent Application Non-Examined Publication No. H2-291624.
FIG. 3A illustrates a plan view, partly in section, showing a conventional thermal fuse. FIG. 3B illustrates a sectional view taken along the line 3B—3B in FIG. 3A.
As shown in FIGS. 3A and 3B, a conventional thermal fuse has a pair of metal terminal 1 each mounted on bottom surface of insulation film 2, protruding a portion of metal terminals 1 from bottom surface to out of upper surface of insulation film 2. Fusible alloy 4 is coupled between protruding ends of a pair of metal terminals 1. Fusible alloy 4 is coated with flux 3. Flux coating is applied on fusible alloy 4 by dripping with heat-liquefied flux 3. Insulation cover film 5 is disposed above insulation film 2 to cover fusible alloy 4. Insulation cover film 5 is preferably transparent or translucent enable to see inside conditions of the thermal fuse.
Certain degree of unavoidable fluctuation in coating quantity of flux occurs in dripping with heat-liquefied flux 3 on fusible alloy 4. However, flux 3 can enhance cutting-off performance of fusible alloy 4 when the alloy fuses. Thermal fuse coated with insufficient quantity of flux, therefore, must be rejected in production inspection process as thermal fuse with insufficient flux shows poor characteristics in cutting-off action.
A downsized thermal fuse is especially needed today along with recent development of downsized batteries used in conjunction with thermal fuses. A thermal fuse that can be inspected on flux coating with high accuracy, therefore, is strongly required since visual check can hardly inspect the flux coating quantity in such downsized thermal fuses.
Typically, coating quantity of flux 3 is inspected by image processing method of color data as follows:                1) storing image data of reflected light or transmitted light from a thermal fuse illuminated by a fluorescent lamp using CCD camera or the like, and        2) inspecting coated quantity of flux 3 according to size of coated or non-coated area with flux indicated in color.        
However, in above mentioned conventional thermal fuse, color of flux 3 varies transparent, yellow or dark brawn or the like due to composition fluctuation of raw materials. Color of flux 3 is expressed in “color scale” as an indicator. In general, “color scale” stands for “Gardner color scale” that specifies color degree of an isopropyl alcohol solution containing 30 wt % of flux. Usually the Gardner color scale is called merely as color scale, so hereafter referred to color scale. The smaller in color scale, the closer to transparent, and the larger in color scale, the closer from yellow, brown to dark brown. In a case, when flux 3 has a small color scale, closer to transparent, it is hard to distinguish in color between flux 3 and insulation cover film 5. On the other hand, when flux 3 has a large color scale, closer to dark brown, inspection accuracy decreases to distinguish in color between flux 3 and fusible alloy 4.
As mentioned above, the drawback in conventional art is that image processing method using CCD camera and the like can not inspect flux coating quantity of thermal fuse with high accuracy due to fluctuation in color of flux 3.